This invention relates to a high speed trust bearing assembly, and more particularly to a high speed thrust bearing assembly which can be used in a circular saw.
Circular saws are used in both a commercial and residential environment to cut a variety of items. The blade of a typical circular saw is driven by an electric motor. The motor has extending therefrom an armature shaft that typically rotates at speeds of up to 25,000 rpm. This armature shaft must be axially supported, while allowing it to properly rotate. In the past, this has been done utilizing one of the two bearing assemblies shown in FIGS. 3 and 4.
As shown in FIG. 3, it is known to insert the end of an armature shaft 10 through a cylindrical sleeve bearing 12. Shaft 10 will rotate within sleeve bearing 12 when the saw is in operation. Sleeve bearing 12 is held within a metal casting 14 in a press-fit relationship. Typical metals used for casting 14 include aluminum and zinc. Casting 14 is coupled to a plastic housing 16 with a suitable attaching means, such as screws 18. Shaft 10 has fixed thereon a commutator 20 that rotates shaft 10 through electromagnetic forces. Between commutator 20 and sleeve bearing 12 are one or more washers 22. In this prior art bearing arrangement, sleeve bearing 12 is axially supported within the metal casting 14. In particular, the contacting of commutator 20 with sleeve bearing 12 through washers 22 prevents axial movement of shaft 10 when the motor is in operation. However, the frictional contact between sleeve bearing 12 and commutator 20, acting through washer 22, occurs at a radially outward location relative to the axis of shaft 10. Therefore, the portion of commutator 20 in contact with sleeve bearing 12 will rotate at a high velocity, resulting in a large heat generation. Washers 22 and zinc casting 14 are employed in this bearing arrangement to dissipate this heat.
An alternative prior art bearing arrangement, as best seen in FIG. 4, eliminates the need for casting 14. In this embodiment, sleeve bearing 12 is held in place within plastic housing 16. Again, axial movement of shaft 10 is prevented by engagement of commutator 20 with sleeve bearing 12. The frictional contact between sleeve bearing 12 and commutator 20 still occurs at a radially outward location relative to the shaft center. Therefore, this embodiment also results in substantial heat generation. To dissipate the heat generated, one or more washers 22 can again be added. Because casting 14 has been eliminated from this arrangement, it is necessary to enlarge sleeve bearing 12 in an attempt to dissipate the heat generated. The use of a larger bearing increases the overall cost of the bearing arrangement, and therefore the overall cost of the product. Further, the generated heat, if not properly dissipated, can increase the wear rate experienced by sleeve bearing 12, thus inhibiting the overall performance of the saw. Neither of the above described prior art bearing arrangements provides a structure that can efficiently retain a high-temperature lubricant in the area of contact between the shaft, the commutator and the sleeve bearing. More specifically, lubricant placed in these areas is subject to a high degree of centrifugal force due to the radially outward location of these areas. Additionally, there is no defined structure to retain the lubricant. The use of a high temperature lubricant could further reduce the heat build-up and wear on the bearing assembly.
Therefore, a high speed thrust bearing assembly is needed that is simple in design, easily assembled and that results in lower heat generation so that metal castings and washers can be eliminated from the design. Further, a high speed thrust bearing assembly is needed that does not employ an oversized sleeve bearing. Still further, a high speed thrust bearing assembly is needed that provides a structure to allow a high temperature lubricant to be used to reduce heat and wear within the bearing assembly.